A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macro-cell. In this way, smaller cells, known as micro-cells, pico-cells, or femto-cells, are located within the same coverage area as larger macro cells. The pico-cells and femto-cells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macro-cells are used to provide coverage over a large area, and the smaller underlay cells are used to provide additional capacity in densely populated areas and hotspots, for example. Furthermore, femto-cells can also be used to provide coverage in specific locations such as within a residential home or office. In order to efficiently exploit the additional resource, it is important that any interference between the macro-cell layer and the underlying layer is minimized.
Currently 3rd generation (3G) cellular communication systems based on Code Division Multiple Access (CDMA) technology, such as the Universal Mobile Telecommunication System (UMTS), are being deployed, with a trend towards introducing a large number of femto-cells in these 3G systems. For example, it is envisaged that a Residential Access Point (RAP) may be deployed having a target coverage area of only a single residential dwelling or house. A widespread introduction of such a system would result in a very large number of small underlay cells within a single macro-cell.
Generally, during deployment of a cellular network of macro cells and underlay cells, the planned layout is fixed and known. In a centrally controlled database deployment process, well established techniques exist to calculate the optimal pilot power levels. For example, the underlay access points of such systems receive a neighbour list identifying a number of neighbour cells and the access points measure pilot signal power levels of these neighbour cells. These levels for each neighbour macro-cell is measured and reported back to the central database, such as in a radio network controller (RNC) or Mobile Switching Centre (MSC). The central database then uses these measurements to determine an appropriate pilot power level for that femto-cell. However, a problem arises in the introduction of underlaying cells, such as residential deployments of femto-cells where the deployment process is incremental, unilateral, and ad hoc.
In particular, a UMTS RAP may need to be able to operate on the same frequency as a macro layer cell in the UMTS network. Under these circumstances and without coordination between the RNC of the macro layer and the RAP layer, and considering a much smaller dynamic range of power control at the RAP, there are cases where the RAP suffers interference from macro layer user equipment (UE) in close proximity thereto and vice versa. In the case of inbound interference case to the RAP from the Macro system the RAP suffers problematic noise rises and the only way out of these difficulties is to turn up its power which can, according to specific scenario, cause two subsequent effects; a) a coverage hole in the macro environment due to RAP power spillover that macro UEs may not be able to camp on themselves, and b) cause so much noise rise (if many RAPs are in the vicinity and there are many connected RAP-UEs) at the Macro node-B site that the macro environment becomes unstable itself (i.e. there is a net reduction in macro capacity).
Obviously the RAP needs to be sensitive to macro layer UEs or the RAP will cause a net reduction in coverage in the macro environment. Similar effects, but less dramatic overall, occur with mutual RAP-to-RAP interference cases, However, generally RAPs seek to constantly keeping their powers low to avoid such situations while maintaining adequate power for their UE clients. Additionally, an ad hoc cellular system that consists of femto-cells of residential access points and macro layer mobile user equipment is by definition changeable. No settings, regardless of how optimal, are stable for long, and will change over time.
There are many traditional inter-frequency and power control algorithms defined in the art, however these solution require some amount of coordination or control between the hierarchical layers of the communication system. Even within the one layer of the RAP environment there is no coordination between RAPs.
What is needed is power control optimization on the part of the RAPs that can be performed unilaterally without coordination between the macro layer RNCs, NodeBs, or other RAPs.